1. Field of the Invention
The present invention relates to a ferroelectric memory and a method of manufacturing the ferroelectric memory, and more particularly to a ferroelectric memory comprising a ferroelectric film which is an inorganic dielectric film having a low dielectric constant.
2. Description of the Related Art
A ferroelectric memory which is currently studied is roughly divided into two parts. One of them is a memory using a method of detecting the inversed electric charge quantity of a ferroelectric capacitor and constituted by the ferroelectric capacitor and a selective transistor.
The other is a memory using a method of detecting a change in the resistance of a semiconductor by the spontaneous polarization of a ferroelectric. The method typically includes an MFSFET. The MFSFET has an MIS structure using a ferroelectric for a gate insulating film. With this structure, it is necessary to directly form a dielectric on the surface of the semiconductor and the interface of the ferroelectric/semiconductor is hard to control. For this reason, it is very hard to manufacture a memory element of high quality. Currently, a memory structure having a buffer layer provided on a ferroelectric/semiconductor interface is a mainstream. However, the inventors have proposed an FET having an MFMIS structure in which a metal layer (M) and an insulating layer (I) are provided as buffer layers on the ferroelectric/semiconductor interface as shown in FIG. 4. The FET having the MFMIS structure comprises a gate oxide film 5, a floating gate 6, a ferroelectric film 7 and a control gate 8 which are sequentially provided on a channel region 4 formed between source and drain regions 2 and 3 of a semiconductor substrate 1.
With this structure, usually, when the semiconductor substrate 1 is provided and a positive voltage is applied to the control gate 8, the ferroelectric film 7 causes a polarization inversion. Even if the voltage of the control gate 8 is removed, a negative electric charge is generated in a channel formation region CH by the residual polarization of the ferroelectric film 7. This state is set to be xe2x80x9c1xe2x80x9d.
To the contrary, when a negative voltage is applied to the control gate 8, the ferroelectric film 8 causes a polarization inversion in a reverse direction. Even if the voltage of the control gate 8 is removed, a positive electric charge is generated in the channel formation region CH by the residual polarization of the ferroelectric film 8. This state is set to be xe2x80x9c0xe2x80x9d. Thus, information xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d can be written to the FET.
The written information is read by the application of a reading voltage Vr to the control gate 8. The reading voltage Vr is set to be a value between a threshold voltage Vth1 in the state of xe2x80x9c1xe2x80x9d and a threshold voltage Vth0 in the state of xe2x80x9c0xe2x80x9d. By detecting that a drain current flows or not when the reading voltage Vr is applied to the control gate 8, it is possible to decide whether the written information is xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d.
According to the FET having the MFMIS structure, thus, one memory cell can be constituted by one element and nondestructive readout can be carried out well.
However, such an FET having the MFMIS structure has the following problem. At time of writing, the FET has such a configuration that a capacitor Cf (a capacitance Cf) of the ferroelectric film 7 and a capacitor Cox (a capacitance Cox) of the gate oxide film 5 are connected in series (see FIG. 5). CD represents a drain capacitance and is disregarded herein. In the case in which a voltage V is applied between the substrate 1 and the control gate 8, therefore, the voltage V is divided into Vf and Vox so that the following equation (1) is obtained.
V=Vf+Vox
CfVf=CoxVox=q (q: quantity of electric charge generated by capacitor)xe2x80x83xe2x80x83(1)
Accordingly, a partial pressure Vf expressed in the following equation is applied to the capacitor Cf of the ferroelectric film 7.
Vf=Cox/(Cf+Cox)xc2x7Vxe2x80x83xe2x80x83(2)
On the other hand, it is necessary to increase Vf to some extent in order to carry out a polarization inversion over the ferroelectric film 7 during writing.
Accordingly, it is necessary to reduce the capacitance of the ferroelectric film against the capacitance of the gate insulating film. For example, however, there is a problem in that the relative dielectric constant of PZT is approximately 200 to 1000 and is much higher than a relative dielectric constant of 3.9 of a silicon oxide film constituting the gate insulating film.
For this reason, it is hard to increase the partial pressure Vf in the equation (1). Accordingly, there is a problem in that it is hard to carry out the polarization inversion over the ferroelectric film 7 during writing.
In order to solve the problem, it is necessary to reduce the thickness of a film in order to decrease the relative dielectric constant of the ferroelectric film as much as possible. By reducing the thickness of the film, thus, it is possible to increase the partial pressure Vf. On the other hand, when the thickness of the film is reduced, a leakage current is actually generated between the floating gate and the control gate, causing a deterioration in a memory characteristic.
In order to increase the speed of a ferroelectric memory and to reduce power consumption, thus, it is an important object to reduce a dielectric constant with a decrease in the relative dielectric constant of a ferroelectric film.
The insulating film has variously been devised in order to reduce a dielectric constant. In general, the following methods have conventionally been proposed to reduce the dielectric constant of the insulating film:
(1) to add fluorine to a silica film to be an inorganic insulating film;
(2) to form an organic insulating material having a low dielectric constant as a parent material; and
(3) to intentionally form a porous film.
In the case of the method (1), however, since the heat resistance of the insulating film is deteriorated, the addition is carried out in an element ratio of several % at most. Consequently, there is a problem in that the relative dielectric constant can be reduced by only 10 to 15% of that of a conventional insulating film.
In the case of the method (2), moreover, the organic material is formed. For this reason, there is a problem in that the heat resistance is considerably deteriorated as compared with a conventional silica based insulating film, resulting in a reduction in the reliability of a semiconductor element. Therefore, the organic material cannot be applied to a ferroelectric film at all.
In the case of the (3), furthermore, since a porous structure is random, the mechanical strength of the insulating film is remarkably reduced and the insulating film is apt to be broken in packaging, causing a reduction in the reliability of a semiconductor element.
In many cases, moreover, the porous structure is not closed. If the porous structure is not closed, the moisture resistance of the insulating film is remarkably deteriorated, causing a reduction in the reliability of the semiconductor element.
In the conventional insulating film as well as the ferroelectric film, thus, there is a problem in that the dielectric constant cannot be reduced sufficiently, and furthermore, the mechanical strength is also insufficient.
In consideration of the circumstances, it is an object of the invention to reduce a leakage current and to enhance the data holding characteristic of a ferroelectric memory in order to increase the speed of the ferroelectric memory and to decrease power consumption.
More specifically, it is an object of the invention to provide a ferroelectric film having a low dielectric constant and a high mechanical strength.
The invention is characterized in that a ferroelectric layer is constituted by an inorganic film having a vacancy rate of 50% or more in an FET having an MFMIS structure.
More specifically, the invention provides a first ferroelectric memory comprising an FET having an MFMIS structure in which a floating gate, a ferroelectric layer and a control gate are sequentially provided through a gate insulating film on a surface of a semiconductor substrate between source and drain regions formed on the surface of the semiconductor substrate, wherein the ferroelectric layer is constituted by an inorganic insulating film having a vacancy rate of 50% or more.
According to such a structure, since air has a low dielectric constant, the dielectric constant can further be reduced as compared with the addition of fluorine so that the dielectric constant of the insulating film can be reduced as much as possible. Accordingly, a polarization inversion voltage can be dropped and a driving voltage can be reduced. Moreover, since the film has a great mechanical strength and a high reliability, a leakage current between the floating gate and the control gate can also be reduced.
It is desirable that the ferroelectric film should be constituted by STN(Sr2(Ta1xe2x88x92xNbx)2O7)x: 0 less than x less than 1.
In the STN, an ordinary material itself has a relative dielectric constant of approximately 40 to 50. By setting the vacancy rate to 50% or more, the relative dielectric constant can be reduced to approximately 20 to 25 or less. Consequently, it is possible to reduce a leakage current without greatly dropping a voltage to be applied to the ferroelectric film.
It is desirable that the vacancy of the inorganic insulating film should have a degree of orientation.
According to such a structure, the vacancy has the degree of orientation and a periodic porous structure. Therefore, the mechanical strength can be increased so that a ferroelectric film having a high reliability can be obtained.
Moreover, it is desirable that the inorganic insulating film should be formed on the surface of the substrate and should have a periodic porous structure including a cylindrical vacancy orientated in parallel with the surface of the substrate.
According to such a structure, since the vacancy is oriented in parallel with the surface of the substrate, a low dielectric constant can be uniformly obtained in a direction perpendicular to the surface of the substrate. Moreover, it is possible to serve as an effective thin film having a low dielectric constant in which a moisture resistance is excellent and a reliability is high.
It is desirable that there should be a plurality of periodic porous structure domains including a cylindrical vacancy oriented in one direction in parallel with the surface of the substrate, and the adjacent porous structure domains should be oriented in different directions from each other.
According to such a structure, the porous structure is oriented in a different direction for each domain. Therefore, the opening portions of the vacancies can be closed each other, and it is possible to obtain a thin film having an extremely low dielectric constant in which a moisture resistance is almost as high as that of a fine film and the periodic structure can also give a high mechanical strength. Furthermore, an interlayer space is supported by an adjacent layer. Consequently, a layered periodic porous shape which is usually supposed to be unstable can be taken with a stable and high mechanical strength.
The invention provides a method of manufacturing a ferroelectric memory comprising an FET having an MFMIS structure in which a floating gate, a ferroelectric layer and a control gate are sequentially provided through a gate insulating film on a surface of a semiconductor substrate between source and drain regions formed on the surface of the semiconductor substrate, wherein a process for forming the ferroelectric layer comprises a step of producing a precursor solution containing a derivative and a surfactant, a preliminary crosslinking step of raising a temperature of the precursor solution and starting a crosslinking reaction, a contact step of causing the precursor solution starting a crosslinking reaction at the preliminary crosslinking step to come in contact with the surface of the substrate, and a step of burning the substrate with which the precursor solution is caused to come in contact and decomposing and removing the surfactant.
According to such a structure, it is possible to provide an insulating film having a very excellent controllability, a high mechanical strength and an extremely low dielectric constant.
Moreover, it is possible to properly change the vacancy rate by regulating the concentration of a precursor solution. Thus, it is possible to form an insulating thin film having a very high workability and a desirable dielectric constant.
It is desirable that the substrate should be immersed in the precursor solution at the contact step.
According to such a structure, it is possible to form an insulating film having a low dielectric constant with a high productivity.
Moreover, it is desirable that the substrate should be immersed in the precursor solution and should be pulled up at a desirable speed in the contact step.
According to such a structure, it is possible to form an insulating film having a low dielectric constant with a high productivity.
It is desirable that the substrate should be coated with the precursor solution at the contact step.
According to such a structure, it is possible to form an insulating film having a low dielectric constant with a high productivity.
It is desirable that the contact step should be a spin coating step of dropping the precursor solution onto the substrate and rotating the substrate.
According to such a structure, the thickness of the film and the vacancy rate can be regulated easily so that an insulating film having a low dielectric constant can be formed with a high productivity.